The ever increasing use of wood as a fuel has resulted in warnings to the public of the danger of chimney fires unless chimneys are cleaned from time-to-time.
When a chimney needs to be cleaned is, at the present time, essentially a matter of guess work unless it has been inspected by a qualified person. Many variables determine the rate at which creosote will build up in a chimney among which, by way of examples, are the frequency with which a chimney is used, whether or not the wood is properly seasoned and whether or not the stove, heater or furnace is of the air tight type.
Unless one were to incur the expense of chimney inspections on a regular basis, there is, as far as I am aware, now no way to determine when a chimney should be inspected and probably cleaned.